Field of the Disclosure
The disclosure generally relates to methods and related apparatus for the measurement and determination of the quality of a foamed asphalt binder. Various asphalt binder/foam quality indicators can be measured with the disclosed methods and apparatus, such as bubble size distribution, bubble surface area, foam expansion ratio, foam half-life, foam index, as well as normalized parameters related to the same.
Brief Description of Related Technology
Interest in Warm Mix Asphalt (WMA) technologies in the U.S. has significantly increased since the first trial in 2004 in Florida and North Carolina. WMA mixing temperatures are 20 to 30° C. lower than conventional Hot Mix Asphalt (HMA) and slightly above 100° C. This amounts to about 20-30% reduction in paving temperatures, which leads to numerous benefits. Lower production temperature reduces the energy consumption, leads to lower greenhouse emissions and less aging of the binder. WMA's advantages (e.g., reduced fuel cost and greenhouse emissions) over traditional HMA has made it very appealing to state and local roadway agencies as well as the industry. In 2010, just six years after the first trial in the U.S., 13% of the pavements were constructed with WMA. When fully implemented, reduction in emissions because of the WMA technology is estimated to be equivalent to removing 1.5 million cars every year.
WMA technologies significantly reduce the mixture viscosity at lower temperatures, facilitate coating and increase the workability. WMA also improves the compaction in cold weather, extends the allowable compaction period (increase the haul distance) and potentially more suitable for using high percentage of reclaimed asphalt pavement (RAP). There are currently over 30 different WMA technologies being used throughout the U.S., where about two thirds of the technologies are based on foamed asphalt. In addition, foaming is the most cost effective WMA technology if the long term production is considered. The working principle of most foamers is to introduce air and water into hot asphalt binder (>100° C.) with the help of a spraying nozzle. The water is turned into steam and expands the binder, significantly decreasing the overall viscosity. This aids in aggregate coating, mixture workability and compaction at lower temperatures.
Currently, there are numerous different field and laboratory foaming techniques being used by the industry. The field foaming technologies can be divided into four major types: (I) foaming nozzle-based methods, (II) synthetic zeolite-based methods, (Ill) indirect foaming via mixing hot aggregates with asphalt and wet fine aggregate, and (IV) shear-based mixing. All of these technologies utilize very different methods. For example, in a laboratory foamer, binder flows down in a pipe (via gravity) around an air/water injection nozzle and hits the injected air/water mixture, which creates the steam bubbles. Whereas, in a field method (ASTEC DBG) air/water mixture (at different concentrations) are sprayed into binder to create foam, then the foam is forced through a narrow nozzle before mixing with aggregate.
On the other hand, in Low Emission Asphalt (LEA), coarse aggregate is heated to about 150° C., mixed with (unfoamed) binder along with a coating/adhesion additive. Then, cold wet fine aggregate and recycled asphalt pavement (RAP) are added. While mixing, the moisture in the wet fine aggregate turns into steam and creates the foam.
Because of a wide variety of methods, the resulting WMA mixture is produced at very different conditions (i.e., temperature, water content, asphalt absorption by aggregates, etc.). As a result, parameters affecting WMA pavement performance such as degree of coating, amount of trapped moisture, and asphalt binder absorption onto aggregates may exhibit great variation. However, there is no clear understanding of the effects of different WMA technologies on the quality of the foam generated, which can significantly affect the overall global performance of the mixture. This lack of understanding is partly because there is no standard test method for measuring the characteristics (i.e., the quality) of foamed binders.